1. Field of the Invention
The present invention relates to a multiplex drive liquid crystal display device using a field effect type twisted nematic effect.
2. Description of the Related Art
A liquid crystal display apparatus having a plurality of pixels arranged in a matrix manner is applied to a display unit of a computer terminal, an image display unit of a television receiver, and the like. Recently, a demand has arisen for a large size and high image quality of the image display unit. Therefore, an increase in number of pixels and an improvement in contrast are desired. A liquid crystal display apparatus applied to the image display unit has a simple matrix type twisted nematic liquid crystal display device (to be referred to as a matrix TN. LC. device) arranged such that a plurality of electrodes are aligned on inner surfaces of a pair of opposing substrates and opposing portions of the electrodes form a plurality of pixels aligned in a matrix manner. The matrix TN. LC. device is driven in a multiplexed manner.
In the matrix TN. LC. device, if the number of pixels is increased in order to improve the resolution and increase a display area, the number of scanning lines is naturally increased. Therefore, high multiplex drive must be performed. However, if a multiplexing degree is increased, a difference in effective voltages between an on electric field to be applied to a liquid crystal to turn on pixels and an off electric field to be applied to the liquid crystal to turn off the pixels is reduced. As a result, an operating margin of a drive voltage is reduced, the contrast is lowered, and a viewing angle characteristic is degraded.
The operating margin and the contrast of a liquid crystal display device depend on a voltage-luminance characteristic. That is, when a change in transmittivity with respect to a change in intensity of the electric field to be applied to the liquid crystal is steep, the operating margin can be increased, and the contrast can be increased. As shown in FIG. 1, the steepness of the voltage-luminance characteristic is represented by a ratio (to be referred to as .gamma. value hereinafter) between voltage V.sub.50 at which the transmittivity is 50% and threshold voltage V.sub.C. When the .gamma. value becomes closer to 1, the change in transmittivity becomes steeper. Therefore, the operating margin can be increased, and the contrast can be increased.
In addition, in the matrix TN. LC. device which is of high multiplex drive type, a multiplexing degree is increased, and one selection period is shortened. Therefore, the matrix TN. LC. device must respond at high speed.
As described above, the matrix TN. LC. device of high multiplex degree must have:
(1) a .gamma. characteristic close to 1; PA1 (2) a wide viewing angle; and PA1 (3) a high response speed. PA1 V.sub.C : the threshold voltage PA1 K.sub.11 : the splay elastic constant of the liquid crystal PA1 K.sub.33 : the bending elastic constant of the liquid crystal PA1 .DELTA..epsilon. : the dielectric anisotropy of the liquid crystal PA1 .epsilon..sub..perp. : the dielectric constant in a direction perpendicular to a liquid crystal molecular axis PA1 .DELTA.n : the optical anisotropy of the liquid crystal PA1 d : the liquid crystal layer thickness PA1 .lambda. : the wavelength of light PA1 (d) .DELTA.n.multidot.d must be reduced; and PA1 (e) .DELTA.n must be reduced. PA1 .eta. : viscosity PA1 .epsilon..sub.0 : dielectric constant in vacuum PA1 E : electric field intensity PA1 K.sub.22 : a twist elastic constant PA1 (f) viscosity .eta. must be reduced; and PA1 (g) the electric field intensity must be increased. PA1 a pair of opposing substrates; PA1 a plurality of electrodes arranged on opposing inner surfaces of the pair of substrates so as to oppose each other; PA1 aligning films formed on the inner surfaces of the pair of substrates on which the plurality of electrodes are formed, and oriented in a predetermined direction; PA1 a nematic liquid crystal, interposed between the opposing electrodes formed on the pair of opposing substrates, and having a value of dielectric ratio .DELTA..epsilon./.epsilon..perp. of 0.5 or less which is a ratio of dielectric anisotropy .DELTA..epsilon. to dielectric constant .epsilon..perp.in a direction perpendicular to a liquid crystal molecular axis direction and a value of elastic constant ratio K.sub.33 /K.sub.11 of 0.8 or less which is a ratio of bending elastic constant K.sub.33 to splay elastic constant K.sub.11 ; and PA1 a pair of polarizing plates arranged to sandwich the nematic liquid crystal.
The .gamma. characteristic is studied by M. Schadt et al. According to their studies, the .gamma. value representing the steepness of the voltage-luminance characteristic is given by the following equation (I) and coincides well with the characteristic of an actual device: ##EQU1## where V.sub.50 : the applied voltage when a transmittivity of 50% is obtained
According to equation (I), it is apparent that when the first, second, and third terms of equation (I) are close to 1 the .gamma. value is close to 1. Therefore, in order lo improve the .gamma. value characteristic, the following conditions must be simultaneously satisfied:
(a) a ratio (to be referred to as elastic constant ratio K.sub.33 /K.sub.11 hereinafter) of bending elastic constant K.sub.33 to splay elastic constant K.sub.11 is small;
(b) a ratio (to be referred to as dielectric ratio .DELTA..epsilon./.epsilon..perp. hereinafter) of dielectric anisotropy .DELTA..epsilon. to the dielectric constant in a direction perpendicular to the liquid crystal molecular axis, is small; and
(c) a value of product .DELTA.n.multidot.d of liquid crystal optical anisotropy .DELTA.n and liquid crystal layer thickness d is 1.1 (.mu.m) when a wavelength of incident light is 550 nm.
Dependency (to be referred to as a viewing angle characteristic hereinafter) of the contrast to an observing direction is studied by Mr. G. BAUR and reported in "The Influence of Material and Device Parameters on the Optical Characteristics of Liquid Crystal Displays", Molecular Crystals and Liquid Crystals, Volume 63, Nos. 1 to 4, 1981. According to this report, the viewing angle characteristic of a liquid crystal display device depends on liquid crystal layer thickness d and liquid crystal optical anisotropy .DELTA.n of a liquid crystal. That is, in a liquid crystal display device having large product .DELTA.n.multidot.d (to be referred to as .DELTA.n.multidot.d hereinafter) of layer thickness d and optical anisotropy .DELTA.n, an apparent change rate of .DELTA.n.multidot.d obtained when the liquid crystal display device is viewed from its front and in an oblique direction is large, resulting in a poor viewing angle characteristic. To the contrary, a liquid crystal display device having small .DELTA.n.multidot.d has a good viewing angle characteristic. In addition, when liquid crystal display devices having equal .DELTA.n.multidot.d are compared, a better viewing angle characteristic is obtained with smaller optical anisotropy .DELTA.n of the liquid crystal. That is, a better viewing angle characteristic is obtained when a change in contrast with respect to a change in observing direction is small. Therefore, in order to improve the viewing angle characteristic:
As for the response characteristic, response time t.sub.ON required for turning on the liquid crystal display device and response time t.sub.OFF required for turning off the liquid crystal display device are represented by the following logic equations (II) and (III), respectively, and coincide well with measurement values: EQU t.sub.ON =.eta./(.epsilon..sub.0 .DELTA..epsilon.E.sup.2 -Kq.sup.2)(II) EQU t.sub.OFF =n/Kq.sup.2 (III)
where q=.pi./d, K=K.sub.11 +[(K.sub.33 -2K.sub.22)/4]
According to equations (II) and (III), the response speed depends on viscosity .eta. and electric field intensity E. That is, in order to increase the response speed:
Of the above conditions (a) to (c) for obtaining a steep .gamma. characteristic, the condition of .DELTA.n.multidot.d has a largest influence on the .gamma. characteristic. Therefore, in consideration of the above technical background, a value of .DELTA.n.multidot.d of a conventional matrix TN. LC. device is set to be about 1.1 (.mu.m) because the center of a wavelength range of a visual light beam is about 550 nm. In this case, since optical anisotropy .DELTA.n of a liquid crystal generally falls within the range of 0.13 to 0.16, liquid crystal layer thickness (interelectrode gap) d is set to fall within the range of 7.0 to 8.5 (.mu.m).
The above conventional liquid crystal display device has a .DELTA.n.multidot.d value of 1.1 and therefore has a relatively high contrast. However, the contrast is not sufficient yet. In addition, since the value of .DELTA.n.multidot.d is large, the viewing angle characteristic is poor, and the response speed is low because liquid crystal layer thickness d is increased. If thickness d is reduced in order to increase the response speed, the value of .DELTA.n must be increased to be 0.147 or more to satisfy the condition of .DELTA.n.multidot.d for obtaining a high contrast. In this case, however, the viewing angle characteristic is further degraded because the value of .DELTA.n is large. If the values of thickness d and .DELTA.n are reduced in order to improve the viewing angle characteristic and the response speed, the value of .DELTA.n.multidot.d largely becomes different from an optimal value of 1.1, and the contrast is significantly lowered. Therefore, a high contrast, a wide viewing angle, and a high-speed response cannot be satisfied.
As described above, according to the conventional liquid crystal display device, it is difficult to obtain an image display with sufficiently high contrast and to satisfy above conditions (1) to (3) required for the matrix TN. LC. device of high multiplex drive type. As a result, an image display cannot be obtained with sufficiently high display quality.